Flaws in the oxide layer of a field effect transistor (FET) device present one of the most important reliability problems for an integrated circuit (IC). Point defects can develop during oxidation processes. Defects may take the form of particulate impurities or pinholes. If the defects are large enough, shorts are created between a gate layer and substrate. Flaws can also present problems in the absence of a short. The local breakdown voltage for the oxide layer in the vicinity of a defect can be considerably lower than the theoretical breakdown voltage for the oxide material. For example, a defect having a size of about half of the oxide thickness may reduce the breakdown voltage by about 50%.
To screen out gate oxide defects, IC manufacturers perform gate stress and burn-in procedures to improve IC reliability. During a typical gate stress procedure, a voltage in excess of the normal operating voltage but less than the maximum rated voltage is applied to the gates of the FET devices in an IC. Gate current leakage is measured before and after the stress. Detection of increases in gate leakage, parametric circuit shifts, or ruptured gate oxide layers are indicative of defects, and used to reject devices as failures.
Gate stress procedures are often incompatible with ICs. In single thickness gate oxide ICs, such as a microprocessor, gate stress is often applied to all devices by elevating the supply voltage. But this technique is not effective for mixed-signal and power ICs with multiple thickness gate oxides (e.g. dual dielectric) with both logic and power devices present on the IC. Different voltage values are used to gate stress low voltage logic transistors and power transistors. In other types of circuits, such as analog circuits, the nature of the analog circuit topology may prevent all components from being gate stressed.
Burn-in procedures are expensive and do not always screen out gate oxide defects. Implementing a burn-in procedure often takes an undesirably long period of time. The burn-in procedure also typically involves costly equipment. A considerable amount of manufacturing floor space is devoted to the equipment. Burn-in costs often amount to a significant component of the overall total cost of an IC product.